There exists a need to create three-dimensional micro-structures in many applications including processing integrated circuits, trimming thin film heads for disk drives, processing Micro Electro Mechanical Systems (MEMS), and preparing work pieces for viewing in Transmission Electron Microscopes (TEMs). These applications and others continually demand increased speed and accuracy for creating ever smaller, more elaborate, microscopic features on solid surfaces. Focused ion beam (“FIB”) technology, with its ability to both remove and deposit material on the sub-micron size scale, is often used to perform three dimensional device modifications. Typically, the FIB is used to either remove or deposit a thin layer of material. With many applications, many cubic microns of material must be removed or added in seconds or minutes.
The standard methods for creating microscopic features involve scanning a finely focused ion beam in a raster type pattern over the target surface to mill, etch, or deposit material. The target area is generally divided into a matrix of virtual dwell points (or pixels) arranged in a grid like pattern. The beam is scanned over each line of virtual dwell points, one line at a time. As it is scanned in this way, its intensity is modulated (e.g., turned on or off) as the beam passes over a pixel so that each pixel receives an appropriate “shot” from the beam. In this way, a desired two-dimensional pattern is achieved once all of the lines of pixels have been scanned.
By so directing beam particles onto a target, one can either mill or etch the object or deposit material onto it. Milling involves the direct removal of surface material by the impact of ions in a process called sputtering. Etching, assisted by the ion beam, can be done by introducing a reactive vapor that can volatize the target material and thereby increase the speed of this process. In FIB deposition, a gas, typically including organometallic compounds, is directed toward the impact point of the FIB on the target surface. The gas decomposes in the presence of the ion beam to add material to the target surface. Ion beam assisted deposition processes are described, for example, in U.S. Pat. No. 4,876,112 to Kaito et al. for “Process for Forming Metallic Patterned Film,” and U.S. Pat. No. 5,827,786 to Puretz for “Charged Particle Deposition of Electrically Insulating Films.”
Fully three dimensional structures are generally created by considering the shape of the desired structure in thin slices parallel to the surface. One either etches into the surface or deposits onto it, one slice at a time, by defining an appropriate raster-type scan operation for each slice. The process is continued until the entire structure is generated. Such a process is described, for example, in U.S. Pat. No. 5,389,196 for “Methods for Fabricating Three-Dimensional Microstructures.”
Unfortunately, with this conventional approach, it is important that the beam be smaller (in cross section) than any of the facets of the structure to be created so that they are created with sufficient fidelity. In many applications, this may require beams that are only several nanometers in diameter. Such beams can be costly if not impossible to produce. In addition, only certain types of beam generation methods with certain types of beam sources (e.g., gallium ion source) may be available. This can be problematic when working with objects made of certain materials that may adversely react with such beam types.
Moreover, beams having very small cross-sections typically have fewer ions (reduced beam current), so processing time for smaller beams is longer. To make up for the smaller diameters, beam current densities for these sharper beams are typically increased in order to retain sufficiently high overall beam current for minimizing processing time. This, however, has its own associated problems. With higher beam current densities, as the ion beam dwells on each virtual dwell point in its scan pattern, adsorbed gas molecules are reacted and removed faster than they can be replenished by the gas jet. This phenomena is known as “overmilling” and applies to both FIB etching and depositing when the gas flux is insufficient to support the ion flux. This extensive gas removal makes the ion beam induced etch or deposition less efficient than if a higher density of adsorbed molecules were present on the surface. In deposition, the low density of the adsorbed gas not only reduces the deposition rate, but also some of the material already deposited may be etched away by the ion beam.
Accordingly, what is needed is an improved method and system for creating three-dimensional micro-structures.